1. Technical Field
The present invention relates to an image forming apparatus provided with an image bearing member that bears an image and a transfer roller that makes contact with the image bearing member and that has a concave portion in its circumferential surface, and to a method for forming an image using such an image forming apparatus.
2. Related Art
In the field of image forming techniques for forming images upon a recording material such as paper, there are apparatuses configured so as to form a transfer nip by bringing a transfer roller into contact with an image bearing member that temporarily bears an image, and transferring the image onto the recording material by causing the recording material to pass through the transfer nip. For example, with a liquid developer-type image forming apparatus disclosed in JP-A-2002-156830 (for example, see FIG. 1), a backup roller is brought into contact with a drum-shaped intermediate transfer medium at a constant load, and a toner image upon the intermediate transfer medium is pressure-transferred onto paper by causing that paper to pass through a nip thus formed.
When a contact pressure applied to the recording material at the transfer nip increases, so does the likelihood that a problem in which the recording material sticks to the image bearing member will occur. In order to prevent this problem, it is conceivable to provide a concave portion in the transfer roller by partially cutting out the circumferential surface of the transfer roller, and provide a gripping member that grips an end of the recording material within that concave portion. However, if such a measure is taken, the circumferential surface of the transfer roller will no longer have a perfect cylindrical surface, and as a result, the state of contact with the image bearing member will fluctuate in a cyclical manner as the transfer roller rotates. Accordingly, a torque for rotating the transfer roller fluctuates in accordance with the rotation of the transfer roller. Such a fluctuation in the torque causes fluctuations in the speeds of the transfer roller and the image bearing member, which in turn inhibits stable image formation on the image bearing member.